148 research outputs found
High efficient luminescence in type-II GaAsSb-capped InAs quantum dots upon annealing
The photoluminescence efficiency of GaAsSb-capped InAs/GaAs type II quantum dots (QDs) can be greatly enhanced by rapid thermal annealing while preserving long radiative lifetimes which are ∼20 times larger than in standard GaAs-capped InAs/GaAs QDs. Despite the reduced electron-hole wavefunction overlap, the type-II samples are more efficient than the type-I counterparts in terms of luminescence, showing a great potential for device applications. Strain-driven In-Ga intermixing during annealing is found to modify the QD shape and composition, while As-Sb exchange is inhibited, allowing to keep the type-II structure. Sb is only redistributed within the capping layer giving rise to a more homogeneous composition
Modelling of bismuth segregation in InAsBi/InAs superlattices: Determination of the exchange energies
InAsBi dilute alloys are potential new candidates for the improvement of infrared optoelectronic devices such as photodetectors or lasers. In this work, InAsBi/InAs superlattices (SLs)with Bi contents ranging between 1 and 3% were grown by molecular beam epitaxy with different Bi fluxes and growth temperatures to analyze Bi segregation by cross sectional transmission electron microscopy techniques. Bi segregation profiles have been described layer-by-layer using a three-layer fluid exchange mechanism, extracting the values of the As/Bi exchange energies (E 1 , 1.26 ± 0.01 eV and E 2 , 1.36 ± 0.02 eV). A relationship to calculate the activation energies for exchange from the binding energies in III–V alloys is proposed, which would allow predicting them for other hitherto unknown compounds
Evaluation of the In desorption during the capped process in diluted nitride In(Ga)As quantum dots
Diluted nitride self-assembled In(Ga)AsN quantum dots (QDs) grown on GaAs substrates are potential candidates to emit in the windows of maximum transmittance for optical fibres (1.3-1.55 μm). In this paper, we analyse the effect of nitrogen addition on the indium desorption occurring during the capping process of InxGa1−xAs QDs (x = l and 0.7). The samples have been grown by molecular beam epitaxy and studied through transmission electron microscopy (TEM) and photoluminescence techniques. The composition distribution inside the dots was determined by statistical moiré analysis and measured by energy dispersive X-ray spectroscopy. First, the addition of nitrogen in In(Ga)As QDs gave rise to a strong redshift in the emission peak, together with a large loss of intensity and monochromaticity. Moreover, these samples showed changes in the QDs morphology as well as an increase in the density of defects. The statistical compositional analysis displayed a normal distribution in InAs QDs with an average In content of 0.7. Nevertheless, the addition of Ga and/or N leads to a bimodal distribution of the Indium content with two separated QD populations. We suggest that the nitrogen incorporation enhances the indium fixation inside the QDs where the indium/gallium ratio plays an important role in this process. The strong redshift observed in the PL should be explained not only by the N incorporation but also by the higher In content inside the QD
Effect of annealing in the Sb and In distribution of type II GaAsSb-capped InAs quantum dots
Type II emission optoelectronic devices using GaAsSb strain reduction layers (SRL) over InAs quantum dots (QDs) have aroused great interest. Recent studies have demonstrated an extraordinary increase in photoluminescence (PL) intensity maintaining type II emission after a rapid thermal anneal (RTA), but with an undesirable blueshift. In this work, we have characterized the effect of RTA on InAs/GaAs QDs embedded in a SRL of GaAsSb by transmission electron microscopy (TEM) and finite element simulations. We find that annealing alters both the distribution of Sb in the SRL as well as the exchange of cations (In and Ga) between the QDs and the SRL. First, annealing causes modifications in the capping layer, reducing its thickness but maintaining the maximum Sb content and improving its homogeneity. In addition, the formation of Sb-rich clusters with loop dislocations is noticed, which seems not to be an impediment for an increased PL intensity. Second, RTA produces flatter QDs with larger base diameter and induces a more homogeneous QD height distribution. The Sb is accumulated over the QDs and the RTA enlarges the Sb-rich region, but the Sb contents are very similar. This fact leaves the type II alignment without major changes. Atomic-scale strain analysis of the nanostructures reveal a strong intermixing of In/Ga between the QDs and the capping layer, which is the main responsible mechanism of the PL blueshift. The improvement of the crystalline quality of the capping layer together with higher homogeneity QD sizes could be the origin of the enhancement of the PL emission
Effect of MBE growth conditions on GaAsBi photoluminescence lineshape and localised state filling.
A series of gallium arsenide bismide device layers covering a range of growth conditions are thoroughly probed by low-temperature, power-dependent photoluminescence measurements. The photoluminescence data is modelled using a localised state profile consisting of two Gaussians. Good agreement with the raw data is achieved for all layers whilst fixing the standard deviation values of the two Gaussians and constraining the band gap using X-ray diffraction data. The effects of growth temperature and bismuth beam equivalent pressure on the localised state distributions, and other model variables, are both shown to be linked to emission linewidth and device properties. It is concluded that bismuth rich surface conditions are preferable during growth in order to produce the narrowest emission linewidths with this material. These results also show how the growth mode of a gallium arsenide bismide layer can be inferred ex-situ from low-temperature photoluminescence measurements
Prospects for Observations of Pulsars and Pulsar Wind Nebulae with CTA
The last few years have seen a revolution in very-high gamma-ray astronomy
(VHE; E>100 GeV) driven largely by a new generation of Cherenkov telescopes
(namely the H.E.S.S. telescope array, the MAGIC and MAGIC-II large telescopes
and the VERITAS telescope array). The Cherenkov Telescope Array (CTA) project
foresees a factor of 5 to 10 improvement in sensitivity above 0.1 TeV,
extending the accessible energy range to higher energies up to 100 TeV, in the
Galactic cut-off regime, and down to a few tens GeV, covering the VHE photon
spectrum with good energy and angular resolution. As a result of the fast
development of the VHE field, the number of pulsar wind nebulae (PWNe) detected
has increased from one PWN in the early '90s to more than two dozen firm
candidates today. Also, the low energy threshold achieved and good sensitivity
at TeV energies has resulted in the detection of pulsed emission from the Crab
Pulsar (or its close environment) opening new and exiting expectations about
the pulsed spectra of the high energy pulsars powering PWNe. Here we discuss
the physics goals we aim to achieve with CTA on pulsar and PWNe physics
evaluating the response of the instrument for different configurations.Comment: accepted for publication in Astroparticle Physic
Unfolding of differential energy spectra in the MAGIC experiment
The paper describes the different methods, used in the MAGIC experiment, to
unfold experimental energy distributions of cosmic ray particles (gamma-rays).
Questions and problems related to the unfolding are discussed. Various
procedures are proposed which can help to make the unfolding robust and
reliable. The different methods and procedures are implemented in the MAGIC
software and are used in most of the analyses.Comment: Submitted to NIM
Implementation of the Random Forest Method for the Imaging Atmospheric Cherenkov Telescope MAGIC
The paper describes an application of the tree classification method Random
Forest (RF), as used in the analysis of data from the ground-based gamma
telescope MAGIC. In such telescopes, cosmic gamma-rays are observed and have to
be discriminated against a dominating background of hadronic cosmic-ray
particles. We describe the application of RF for this gamma/hadron separation.
The RF method often shows superior performance in comparison with traditional
semi-empirical techniques. Critical issues of the method and its implementation
are discussed. An application of the RF method for estimation of a continuous
parameter from related variables, rather than discrete classes, is also
discussed.Comment: 16 pages, 8 figure
Probing quantum gravity using photons from a flare of the active galactic nucleus Markarian 501 observed by the MAGIC telescope
We analyze the timing of photons observed by the MAGIC telescope during a
flare of the active galactic nucleus Mkn 501 for a possible correlation with
energy, as suggested by some models of quantum gravity (QG), which predict a
vacuum refractive index \simeq 1 + (E/M_{QGn})^n, n = 1,2. Parametrizing the
delay between gamma-rays of different energies as \Delta t =\pm\tau_l E or
\Delta t =\pm\tau_q E^2, we find \tau_l=(0.030\pm0.012) s/GeV at the 2.5-sigma
level, and \tau_q=(3.71\pm2.57)x10^{-6} s/GeV^2, respectively. We use these
results to establish lower limits M_{QG1} > 0.21x10^{18} GeV and M_{QG2} >
0.26x10^{11} GeV at the 95% C.L. Monte Carlo studies confirm the MAGIC
sensitivity to propagation effects at these levels. Thermal plasma effects in
the source are negligible, but we cannot exclude the importance of some other
source effect.Comment: 12 pages, 3 figures, Phys. Lett. B, reflects published versio
Evaluación económica del programa MOSCAMED en Guatemala y sus impactos en ese país, México, EE.UU y Belice
Los objetivos del presente trabajo son evaluar la factibilidad económica de la erradicación de la mosca del Mediterráneo del territorio guatemalteco en un periodo de 10 años (2012-2021), revisar sus impactos en dicho país, en Belice, México y Estados Unidos de América, así como hacer un análisis retrospectivo del Programa Moscamed en Guatemala, de 1978 a 2011, y en Belice, de 1992 a 2011
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